The present invention deals generally with materials which exhibit electromagnetic shielding characteristics. More particularly, the present invention concerns materials which are interactive at microwave frequencies.
In many electronic enclosures, it is desirable to seal openings in the chassis so that various electromagnetic noise and signals do not escape into the surrounding environment and so that external signals do not enter the enclosure. For example, that electromagnetic noise can interfere with nearby television and radio equipment to the consternation of consumers.
In the past, that type of electromagnetic noise and associated signals have been controlled by designing enclosures which have openings that are much smaller than the wavelength of the electromagnetic noise involved. Where it is necessary to cover larger openings, such as for access openings, one past practice has been to use a suitable EMI/EMC gasket to surround the opening to provide electrical contact between the cover and the enclosure. In the industry, "EMI" refers to electromagnetic interference, "EMC" refers to electromagnetic control, and "EMS" refers to electromagnetic shielding. Those EMI/EMC gaskets are typically made from a metal-filled or solid metal material such as beryllium copper which is a good conductor and which is arranged to provide an electrical seal that can conform to an irregular surface which makes electrical contact with adjacent metallic surfaces. Those types that have openings or particle separations must be opaque to electromagnetic noise at the frequencies of interest.
Another problem area concerns electromagnetic reflections from buildings and the like that interfere with television and radio signal reception. For example, a group of tall buildings near one another can cause television signal reflections that result in a plurality of images on local television receivers. Such problems have, in the past, been overcome by use of community television and radio antennae which are placed so as to be free of the interfering reflections.
There have also been a number of engineering design problems which have not met with a readily available solution. More particularly, it is sometimes desirable to design a transmission or receiving antenna from a material which is a good reflector of electromagnetic signals but which is also a nonmetallic material so that it is not subject to attack through environmentally generated phenomena such as rust. Likewise, there are other applications for materials which would be good thermal insulators but which would have high reflectivity, or high absorbtivity of incident electromagnetic signals and noise. For example, a thermally insulated enclosure with a coating that is electromagnetically absorbent would find considerable utility in reducing the radio frequency emissions from some kinds of electronic equipment while increasing the acceptable range of environmental operating temperatures.
In short, it can be seen that the need continues to exist for materials which can be tailored to handle various kinds of incident electromagnetic noise and signals.
In considering the various characteristics of materials that might be useful in tailoring those materials to have specific electromagnetic characteristics, it was noted that the chirality of materials has not been the subject of much analysis or experimentation. A chiral material is one which is asymmetric and which exhibits either left-handedness or right-handedness. One example of a known chiral material is sugar. When a polarized light beam passes through a sugar solution, the light beam rotates through an angle which depends on the sugar molecule and its concentration. Heretofore such chirality of substances has been known and studied in connection with optical beams.
For example, in studies performed by I. Tinoco and M. P. Freeman, "The Optical Activity of Oriented Helices", J. Phys Chem. 61, 1196 (1957), experiments were performed on the electromagnetic activity of helices. Helices were selected as large scale models of sugar molecules. The actual helices used were copper springs about 2 centimeters long that were embedded in styrofoam and excited by low gigahertz frequency electromagnetic energy. Styrofoam is transparent to such electromagnetic energy but the helicies are not. Tinoco measured the resulting rotation of the field.
Another early author in the field worked out the electromagnetics of the optical rotation problem. C. F. Bohren, "Light Scattering by an Optically Active Sphere", Chem. Phys. Lett. 29, 458 (1974).
Propagation of electromagnetic waves through chiral media has been analytically examined. D. L. Jaggard, A. R. Mickelson, and C. H. Papas, "On Electromagnetic Waves in Chiral Media", California Institute of Technology, Antenna Laboratory, Report No. 93 (July 1978). Jaggard et al. analytically concluded that an electrically thin slab with randomly oriented helices effects a rotation of the plane of polarization and that both the reflected and transmitted electromagnetic fields can be analytically described.
Later work considered the effect of an electromagnetic field of microwave frequencies on a sphere made of a chiral material. A. Lakhtakia, V. K. Varadan, and V. V. Varadan, "Scattering and Absorption Characteristics of Lossy Dielectric, Chiral, Nonspherical Objects," Applied Optics Vol. 24, No. 23, (Dec. 1, 1985). In addition, the effect of chirality in a dielectric half-space was analytically investigated by A. Lakhtakia, V. V. Varadan, and V. K. Varadan, "A Parametric Study of Microwave Reflection Characteristics of a Planar Achiral-Chiral Interface", IEEE Transactions on Electromagnetic Compatibility, Vol. EMC-28, No. May 2, 1986.
While no such chiral material that is active at microwave frequencies is known to exist in nature, the work of these various authors assumed that such a material could exist and then went on to analyze it mathematically.
Thus, while no material exhibiting chiral properties at microwave frequencies occurs naturally, these studies have examined it as a mathematical curiosity. On a practical level, optical chirality has been used as a tool of analytical chemistry. But at best, any practical use for the chirality of materials in either the optical range of frequencies or the microwave range of frequencies is obscure.
Another category of materials which was considered as having interest in overcoming the problems with known materials in shielding of electromagnetic waves is the possibility of an electrically conductive polymeric material. For example, it is known that siloxane based polymers exhibit some electrically conductive properties. Moreover, it is known that such materials can be used to make electrical connections between two or more adjacent surfaces. However, the literature does not suggest that anyone appreciated the possibility that electrically conductive polymers may have electromagnetic shielding properties.